1. Field of the Invention
The invention relates to radio frequency identification tags or labels and in particular to a cloaking circuit used to assist in the read operations of RFID transponders.
2. Description of the Prior Art
In U.S. Pat. No. 5,963,144, incorporated herein by reference, what is described is an antenna of an RFID tag or label which is disconnected from the balance of the RFID chip by means of a series switch activated in response to a logic command, CLOAK, generated by the RFID chip. Activation of the switch disconnects the antenna of the RFID tag from the remainder of the RFID chip and effects a high impedance resistance across the antenna terminals. An RC circuit is charged by activation of the CLOAK signal and thereafter discharges during a predetermined RC time period as determined by a high impedance series antifuse leakage transistor. The antenna is thus disconnected for a time sufficient to allow the remaining RFID tags in an RF interrogation field to be identified. Meanwhile, during the disconnection of the antenna from the RFID chip and its loading causes its effective absorption and scattering aperture to be reduced to near zero so as to electromagnetically remove the RFID tag from the zone of interrogation during the predetermined time period. Hence, the interrogated tag remains disconnected and noninterferring with the RF field used to interrogate the remaining tags. It is also known to detune the tag""s antenna by at least partially shorting out the antenna, as either a means of signaling or for depleting the energy stored in the antenna resonant structure.
Thus what is taught is disconnecting the front end of the RFID chip even during times when the power to the tag has been removed. This could be accomplished by open circuiting the data path and/or the power input. Unfortunately, this means that while the chip is in the Cloak state, it is impossible for the reader to communicate with the tag.
As an example, a tag with a cloak time of 20 seconds might be interrogated and then cloaked while on a conveyor belt. But it might be necessary to read the label subsequently when the tag has traveled further down the conveyor belt but within the 20 second time period.
What is needed is a circuit and method that allows a tag to be cloaked, but still to be interrogated when it is cloaked.
The invention is defined as an improvement in a cloaked RFID tag having an antenna comprising a switch and a logic circuit coupled to the switch. The logic circuit or gate selectively allows communication of signals through the antenna during normal operation to thereby allow output of a signal from the RFID tag through the antenna and to disable the RFID output during a cloaking period. A receiving connection is provided to the RFID tag so that command signals are continuously receivable notwithstanding cloaking of the RFID tag.
The RFID tag includes an input circuit. The receiving connection is an electrical connection between the antenna and the input circuit which is not interrupted by operation of the switch. The electrical connection comprises a diode coupled between the antenna and the input circuit. The switch is a grounding switch, such as a switching transistor, coupled between the antenna and ground. The switch communicates signals through the antenna by selectively grounding the antenna according to the signals during the normal operation. The logic circuit couples signals to the switch to ground the antenna during the normal operation and isolates signals from the switch during the cloaking operation. Further power is supplied through the antenna to the RFID circuit during the cloaking operation.
The invention further comprises a method for performing the foregoing operations.
The invention now having been summarized, turn to the following drawing in which like elements are referenced by like numerals.